8,126 research outputs found

### Consideration of building a common platform of collaborative learning environment

This paper reports on considerations about a common and basic functions/components for building a collaborative learning environment. We make efforts to specify the technological issues towards the future standardization of this environment through our research experiences. The problem of standardization includes many embarrassed aspects, however it will extend and widen the field of applications possible within the collaborative learning paradigm, and will make possible the usage of the fruits of years of research and individual implementations of the concept of collaborative learning, from many researches, developments and experiences. So we would like to locate this problem as building a common platform

### Charge transfer in heterostructures of strongly correlated materials

In this manuscript, recent theoretical investigations by the authors in the
area of oxide multilayers are briefly reviewed. The calculations were carried
out using model Hamiltonians and a variety of non-perturbative techniques.
Moreover, new results are also included here. They correspond to the generation
of a metallic state by mixing insulators in a multilayer geometry, using the
Hubbard and Double Exchange models. For the latter, the resulting metallic
state is also ferromagnetic. This illustrates how electron or hole doping via
transfer of charge in multilayers can lead to the study of phase diagrams of
transition metal oxides in the clean limit. Currently, these phase diagrams are
much affected by the disordering standard chemical doping procedure, which
introduces quenched disorder in the material.Comment: 14 pages, 9 figures. Invited article for a special issue of JPCM on
Metal Oxide Thin Films; minor changes in the tex

### Magnetized Accretion Inside the Marginally Stable Orbit around a Black Hole

Qualitative arguments are presented to demonstrate that the energy density of
magnetic fields in matter accreting onto a black hole inside the marginally
stable orbit is automatically comparable to the rest-mass energy density of the
accretion flow. Several consequences follow: magnetic effects must be
dynamically significant, but cannot be so strong as to dominate; outward energy
transport in Alfven waves may alter the effective efficiency of energy
liberation; and vertical magnetic stresses in this region may contribute to
"coronal" activity.Comment: to appear in Ap. J. Letter

### On the Axisymmetric Force-Free Pulsar Magnetosphere

We investigate the axisymmetric magnetosphere of an aligned rotating magnetic
dipole surrounded by an ideal force-free plasma. We concentrate on the magnetic
field structure around the point of intersection of the separatrix between the
open and closed field-line regions and the equatorial plane. We first study the
case where this intersection point is located at the Light Cylinder. We find
that in this case the separatrix equilibrium condition implies that all the
poloidal current must return to the pulsar in the open-field region, i.e., that
there should be no finite current carried by the separatrix/equator current
sheet. We then perform an asymptotic analysis of the pulsar equation near the
intersection point and find a unique self-similar solution; however, a Light
Surface inevitably emerges right outside the Light Cylinder. We then perform a
similar analysis for the situation where the intersection point lies somewhere
inside the Light Cylinder, in which case a finite current flowing along the
separatrix and the equator is allowed. We find a very simple behavior in this
case, characterized by a 90-degree angle between the separatrix and the equator
and by finite vertical field in the closed-field region. Finally, we discuss
the implications of our results for global numerical studies of pulsar
magnetospheres.Comment: 31 pages, including 5 figure

### Evolution of the electronic structure across the filling-control and bandwidth-control metal-insulator transitions in pyrochlore-type Ru oxides

We have performed photoemission and soft x-ray absorption studies of
pyrochlore-type Ru oxides, namely, the filling-control system
Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ and the bandwidth-control system
Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, which show insulator-to-metal transition with
increasing Ca and Bi concentration, respectively. Core levels and the O 2$p$
valence band in Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ show almost the same amount of
monotonous upward energy shifts with Ca concentration, which indicates that the
chemical potential is shifted downward due to hole doping. The Ru 4$d$ band in
Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ is also shifted toward the Fermi level ($E_F$) with
hole doping and the density of states (DOS) at $E_F$ increases. The core levels
in Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, on the other hand, do not show clear energy
shifts except for the Ru 3$d$ core level, whose line shape change also reflects
the increase of metallic screening with Bi concentration. We observe pronounced
spectral weight transfer from the incoherent to the coherent parts of the Ru 4d
$t_{2g}$ band with Bi concentration, which is expected for a bandwidth-control
Mott-Hubbard system. The increase of the DOS at $E_F$ is more abrupt in the
bandwidth-control Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$ than in the filling-control
Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$, in accordance with a recent theoretical
prediction. Effects of charge transfer between the Bi 6$sp$ band and the Ru
4$d$ band are also discussed.Comment: 11 pages, 6 figure

### Towards Resolving the Crab $\sigma-$Problem: A Linear Accelerator?

Using the exact solution of the axisymmetric pulsar magnetosphere derived in
a previous publication and the conservation laws of the associated MHD flow, we
show that the Lorentz factor of the outflowing plasma increases linearly with
distance from the light cylinder. Therefore, the ratio of the Poynting to
particle energy flux, generically referred to as $\sigma$, decreases inversely
proportional to distance, from a large value (typically \gsim 10^4)near the
light cylinder to \s \simeq 1 at a transistion distance $R_{\rm trans}$.
Beyond this distance the inertial effects of the outflowing plasma become
important and the magnetic field geometry must deviate from the almost
monopolar form it attains between $R_{lc}$ and $R_{\rm trans}$. We anticipate
that this is achieved by collimation of the poloidal field lines toward the
rotation axis, ensuring that the magnetic field pressure in the equatorial
region will fall-off faster than $1/R^2$ ($R$ being the cylindrical radius).
This leads both to a value \s=\ss \ll 1 at the nebular reverse shock at
distance $R_s$ ($R_s \gg R_{\rm trans}$) and to a component of the flow
perpendicular to the equatorial component, as required by observation. The
presence of the strong shock at $R = R_s$ allows for the efficient conversion
of kinetic energy into radiation. We speculate that the Crab pulsar is unique
in requiring \ss \simeq 3 \times 10^{-3} because of its small translational
velocity, which allowed for the shock distance $R_s$ to grow to values $\gg
R_{\rm trans}$.Comment: 16 pages AASTEX, accepted for publication in the Astrophysical
Journa

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